This invention relates generally to apparatus and methods for sensing physical phenomena and particularly to fiber optic sensors which use interferometers to sense changes in physical phenomena.
Optical fibers are sensitive to a large number of physical phenomena, such as acoustic waves and temperature fluctuations. An optical fiber exposed to such phenomena changes the amplitude, phase, or polarization of light propagating therethrough. Optical fibers have been considered for use as sensing elements in hydrophones, magnetometers, accelerometers and current sensors.
Mach-Zehnder, Sagnac and resonant ring interferometers have been used as sensors. In Mach-Zehnder and Sagnac interferometers, the phase difference between interfering lightwaves varies in response to the variations in the sensed quantity, and thus, by detecting the intensity of light produced by such interfering waves, the magnitude of the sensed quantity may be determined. In addition, the detected intensity varies with the phase difference between the interfering light waves. Typically, the interferometer is biased to operate at the point of maximum slope on a cosine curve, the sensed quantity being detected by measuring fluctuations in intensity from this quiescent operating point.
The resonant ring interferometric sensor uses an all-fiber optic ring resonator. The curve of output power with respect to optical wavelength of the fiber optic ring resonator is in the form of an extremely sharp spike. The maximum slope of the output curve, assuming a resonator having a finesse of about 80, is on the order of 14/rad. The all-fiber optic ring resonator therefore provides a highly accurate fiber optic sensor for measuring physical phenomena which affect the parameters of the light propagating through the fiber.
It is possible to use such sensors to detect acoustic vibrations, thermal fluctuations, variations in electromagnetic fields and other physical phenomena. An acoustic wave incident on the fibers which comprise the sensors causes the optical lengths thereof to vary as a function of the acoustic wave magnitude and frequency. The variation of loop length causes the output power of the resonator to vary in a predictable manner. By detecting the variation in output power, a direct indication of the magnitude and the frequency of the acoustic waves may be obtained.
Arrays of sensors are used in various geophysical exploration and antisubmarine warfare applications. Present sensor arrays include many wires, which make them expensive and difficult to operate.